Intramolecular-intermolecular chain

The radical polymerization of acrylic anhydride(AA) as a typical 1,6-diene has been investigated in detail in terms of cyclopolymerization. Thus, in 1958 Crawshaw and Butler(31) and Jones(32) have independently demonstrated that AA could be polymerized in solution by an alternating intramolecular-intermolecular chain propagation or cyclopolymerization, leading to the formation of saturated, linear polymers consisting exclusively of six-membered ring anhydride structure. Later, Mercier and Smets(33)... 

Intermolecular chain transfer reactions may occur between two propagating polymer chains and result in the termination of one of the chains. Alternatively, these reactions take place by an intramolecular reaction by the coiling of a long chain. Intramolecular chain transfer normally results in short branches ... 

Cyclization results from intramolecular chain transfer to polymer, being also indicative of the extent of intermolecular chain transfer leading to formation of branched (nonreactive in the case of oxetanes) tertiary oxonium ion (cf., Section II.D.3). 

Formation of block copolymers in the sequential polymerization may be affected by chain transfer to polymer. As already discussed, in several systems the intramolecular chain transfer to polymer leads to formation of cyclic fraction. Cyclic macromolecules, being neutral, do not participate in further reaction and constitute the homopolymer fraction in resulting copolymer. Intermolecular chain transfer to polymer may lead to disproportionation, i.e., formation of fraction of macromolecules which do not carry active species ... 

In the second case, the soluble polymer 68 was treated with 10-undecenoyl chloride 72 to form the resin-bound alkene 73 (Scheme 16) [11]. Radical addition of xanthate 74, mediated by lauroyl peroxide (50 mol %), gave the product xanthate 75 and tetralone 76 as a 9 1 mixture, in an overall yield of 70%. By-product 76 is formed by intramolecular addition of the intermediate alkyl radical to the aromatic ring competing with intermolecular chain transfer. It was not possible to achieve complete consumption of 73 in this case possibly due to the decrease in the rate of the intermolecular radical addition step as the alkene was consumed. These experiments show, however, that xan-... 

The a and j3 methylene CHj groups (endo- and exocyclic ones) change their diem-ical environment. In the new environnffint their chemical shifts in the H- and C-NMR spectra differ from the previous ones. The corre ronding ectra cannot, however, discriminate between the intermolecular and intramolecular (backbiting) chain transfer, because in the non-strained rit formed by back-biting. 

The disproportionation reaction of the free radical chain can generate the monomer as a successive process. There are, however, some other issues regarding the propagation for free radical chain reactions. In addition to the "regular" propagation step, different reactions may occur in a so-called transfer step. In this step, the free radical chain reacts with another molecule and generates a different radical chain and a new polymeric molecule. There are two possible types of transfer reactions. The transfer step can be an intermolecular chain transfer or an intramolecular chain transfer. An example of an intermolecular chain transfer is... 

The hypotheses considered so far share the attempt of explaining the ease of formation of the ring structures in cyclopolymerization taking into account only the relative ability of the double bond pendant from the active chain end, and of the double bonds of an unreacted monomer molecule, to reach the reaction site. No attention is paid to the fact that the intramolecular and intermolecular chain propagation reactions are basically different, in the sense that the former involves an entropy decrease due to the loss of some rotational degrees of freedom at the end of the active chain, whereas the latter involves an entropy decrease... 

Although there may be some minor contribution of intermolecular chain transfer, these systematic studies have provided a clearer perspective of the mechanism of the intramolecular chain transfer reaction of propagating acrylate radicals. Nevertheless further investigation was required to provide decisive proof of the mechanism. 

The reaction is carried out in solution in an ether (diethyl, diisopropyl, or di-n-propyl ether), at a temperature that must not exceed 100°. To induce intramolecular cyclization at the expense of intermolecular polymerization, the dinitrile is added in a continuous but slow stream to the stirred reaction medium (the Ruggli-Ziegler dilution principle). The decisive factors are the stationary concentration and thus the rate of reaction of the reactants high dilution favours intramolecular cyclization and disfavors intermolecular chain formation, and this dilution is achieved by adding the cyclizing material gradually into a relatively small reaction space. 

The mechanism of polymerization can be described as an equilibration among these various components in addition to reaction with the monomer, the growing silanolate chain ends react with all siloxane bonds via intramolecular cyclization and intermolecular chain transfer as shown in Scheme 7.18. 

However, the low-temperature oxidation of solid polypropylene (70-110°C) proceeds with alternating intramolecular and intermolecular chain transfer. Intramolecular kinetic of extension chains is limited to small parts of the macromolecule with a favorable set of conformations. As a result, blocks of hydroperoxide can be short. In the solid polypropylene has found about 60% of paired units and about 20% of triads, the share of units with a higher number of hydroperoxide groups is small. It should be noted that in other carbon-chain polymers increases the probability of intramolecular reaction at the high rate of conformational motions. For example, in the polymers with a saturated C-C bond (such as... 

Scheme 19.2 Intermolecular (A) and intramolecular (B) chain-transfer reactions in ROMP.

Intramolecular zwitterion pair formation presupposes a quite flexible molecule chain. So, in contrast, stiff molecules form intermolecular chains, which are a kind of polybetaine. 

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